1. Field of the Invention
The present invention relates to an apparatus and method of handling a circulating body of fluid containing solid waste and water, such as wastewater sludge, to effect their separation. (Although the present invention is described in connection with wastewater sludge, it is to be understood that the invention is applicable to wastewater in general whether containing sludge or otherwise.) Such an apparatus and method has application for achieving suspended solid removal, acidity treatment, upflow anaerobic sludge blanket digestion, polishing treatment, activated sludge treatment, grit separation and removal, hydrolysis of primary and secondary sludge, anaerobic and aerobic treatment of primary and secondary sludge, and suspended solid precipitation. They can also be employed in high performance compact reactors.
2. Discussion of Background
There are many industrial, municipal sanitation and other processes that require the confinement and agitation of large standing bodies of liquid with concurrent heating or cooling to control their temperature. The presence of a solid phase within the liquid generally necessitates a) continuing agitation of it in order to suspend the solid more uniformly and, simultaneously, b) maintaining a substantially uniform optimum temperature throughout the liquid. Sewage digestion systems often require such agitation and temperature control, especially in anaerobic sewage systems where really vigorous agitation in the open atmosphere is not possible. At the same time, direct contact of moving mechanical parts with the body of liquid or the solids dispersed therein are to be avoided, because of the nature of the solids often encountered in such systems. Reliability, ease of servicing and maintenance, and the avoidance of any interference with the biological or chemical processes taking place in the liquid are further important criteria that must be met. For a general understanding of the engineering behind such processes and suitable implementing equipment, see Plant Engineering Directory and Specifications Catalog, .COPYRGT. Technical Publishing 1985, Division K on Environmental Control and Water Pollution Control, pages K-14 to K-23, whose contents are incorporated herein by reference.
Sewage digestion systems treat wastewater sludge in a tank while continuously circulating the tank's contents and discharging treated effluent. Generally, a clarifier is provided with a level area to allow water to flow evenly over the surface of a weir as well as for settlement of larger organisms such as protozoa. The clarifier also has an inlet arranged to prevent the entrance velocity of the treated effluent from causing clarifier "short-circuiting" because a water current is created in the clarifier.
Wastewater biomass sludge may be in the form of either primary sludge (i.e., sludge not yet acted upon by active waste organisms) or secondary active sludge (i.e., sludge already acted upon by active waste organisms). Such active waste organisms are commonly found in such biomass sludges, but are generally too small to settle in a clarifier before the effluent is discharged. On the other hand, bacteria-consuming protozoa are developed naturally in biomass, and have greater density and will settle in the clarifier. Protozoa is commonly found in soil, cattle and other ruminant animals and where organic residues and bacteria appear.
Typical active waste organisms include aerobic bacteria and anaerobic bacteria Aerobic bacteria require dissolved oxygen for sustenance and are incapable of independent movement. Therefore, thorough mixing of water, dissolved oxygen, aerobic bacteria, together with larger bacteria-consuming organisms, is required for effective aerobic treatment.
Anaerobic bacteria, on the other hand, thrive on oxygen from their food supply and anaerobic treatment systems often employ an upflow anaerobic sludge blanket digester (hereinafter "UASB"). Typically, wastewater entering from the bottom of the UASB passes through a granular anaerobic sludge layer (i.e., the blanket), where the anaerobic bacteria digest some of the organic matter present in the wastewater and biogas (a mixture of methane and carbon dioxide) is generated. The biogas bubbles become the propellant for the upward flow of sludge particles attached to the bubbles. A gas collector dome is arranged centrally at the top of the UASB to collect the rising gas and the sludge particles are there degassed. The degassed sludge then travels downwardly because of gravity and is deflected by upper surfaces of inclined baffles in the UASB, causing the solids to settle preliminarily on the surfaces and accumulate, and then to drop off either to recirculate with the upward flow of fresh biogas or else settle to the bottom of the digester. Thus, the upward flow of the gasborne sludge in combination with the return downward flow of degassed sludge creates continuous convection and promotes sludge-wastewater contact without the need for energy-consuming mechanical or hydraulic agitation within the digester.
Conventional UASB construction and operation, however, has been found lacking in at least two respects. On the one hand, it has become clear that municipal wastewater, with its relatively high levels of suspended solids, is not an ideal fluid to be handled by, or treated in, an UASB, because such suspended solids tend to coat surfaces and plug conduits and openings, and, thereby, encourage bacteria to develop within the UASB. By contrast, high strength industrial wastewater often contains relatively high levels of chemical oxygen demand (COD) or biochemical oxygen demand (BOD) and dissolved solids, but relatively low levels of suspended solids, and, therefore, tends to be a more suitable medium for UASB handling.
On the other hand, it is also clear that, to the extent current UASB design necessitates removal of treated or separated effluent from essentially the same zone wherein gasborne sludge is being degassed, deleterious materials that contaminate the effluent can also be removed, because a typical UASB baffle system alone cannot assure effluent freedom from such contaminants.